Paper Number: 4330
Evidence from molybdenum isotopes for a transient and localized nature of free aquatic oxygen around 3 Ga.
Kramers, J.D.1, Beukes, N.1, Gumsley, A.1, Guy, B.2, Naegler, Th.3, Pettke, Th.3, Smith, A.B.1 and Voegelin, A.3
1University of Johannesburg, Department of Geology, Auckland Park 2006, South Africa; jkramers@uj.ac.za
2Mineral Services, SGS South Africa (Pty) Ltd., Booysens, Johannesburg 2091, South Africa
3Institute of Geological Sciences, University of Berne, 3012 Berne, Switzerland
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Evidence of early occurrences of low levels of free molecular oxygen has been reported from banded iron formations in the West Rand Group [1] (lower Witwatersrand Supergroup) as well as at the base of its more distal stratigraphic equivalent [2], the Mozaan Group. Here oxygenated conditions have been documented by Cr isotopes on a paleosol [3] and Mo isotopes in banded iron formation [4]. Mo systematics are a powerful in the study of oxygenation in early Earth history. In organic-rich shales and manganese oxide crusts, the combination of Mo concentrations >2 ppm and δ98/95Mo variations > 0.3‰ signal free oxygen at levels as low as 10-5 PAL or less [5], as long as these are present in a sufficiently large reservoir [6]. In order to explore the marine redox conditions at c. 2.95 Ma in a broader stratigraphic and spatial context, we have determined Mo contents and δ98/95Mo values of 20 (mainly carbonaceous) mudrock, banded iron formation (BIF) and manganese concretion samples throughout the West Rand Group as well as 3 samples from the Booysens Formation, Central Rand Group. Further, we analysed 8 similar samples from the Ntombe, Delfkom and Bangaspoort Formations in the Mozaan Group. Mn concretions occur as Mn2+ carbonate, but reflect original precipitation as MnO2 [7], which requires some free O2 even if bacterially mediated [8].
In mudrocks throughout the West Rand Group, we found Mo concentrations varying from 0.2 to 3 ppm and δ98/95Mo values from 0.1 to 0.45‰. Mostly, higher concentrations coincide with higher δ98/95Mo values. A carbonaceous mudrock sample from the Booysens Formation has 8 ppm Mo and δ98-95Mo of 0.6‰. There is thus a weak but persistent signal that free O2 affected the Mo isotopic mass balance in the shelf environment during West Rand Group times (2970-2910 Ma), and in the overlying Booysens Formation this signal is stronger. Three BIF samples from the lowermost West Rand Group and the Booysens Formation have <2ppm Mo but δ98/95Mo from -0.1 to +0.5‰. However, two Mn-concretions from the Brixton Formation have ∼1 ppm Mo and δ98-95Mo of 0.2‰. The samples from the Mozaan Group (mudrocks and Mn concretions alike) present Mo concentrations varying only between 0.2 and 1.5 ppm, with δ98/95Mo between 0.2 and 0.4‰, within the detrital range and thus not indicating free O2. The Mn concretion data present an apparent paradox, since free O2 is required for their formation [8]. However, this requirement is local only, whereas oxygenation on a basin-wide scale would be required for dissolved Mo to reach more distal sedimentation sites. We conclude that free O2 was confined to ‘oxygen oases’ [4]. Further, an episode of strongly oxidizing conditions found at the base of the Mozaan Group [3,4] appears to have been transient.
References:
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[7] Smith AJB et al. (2014) Goldschmidt Conf 2014 Abstracts: 2332.
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